FIG. 1 depicts a schematic diagram of a portion of a wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is a wireless switching center ("WSC"), which also may be known as a mobile switching center or mobile telephone switching office. Typically, a wireless switching center (e.g., WSC 120) is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic region serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). A wireless switching center is responsible for, among other things, establishing and maintaining a call between a first wireless terminal and a second wireless terminal or, alternatively, between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon. In practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises: (1) the radios and antennas that the base station uses to communicate with wireless terminals in that cell; and (2) the transmission equipment that the base station uses to communicate with the wireless switching center.
For example, when a user of wireless terminal 101-1 desires to transmit information to a user of wireless terminal 101-2, wireless terminal 101-1 transmits a data message bearing the user's information to base station 103-1. The data message is then relayed by base station 103-1 to wireless switching center 120 via wireline 102-1. Because wireless terminal 101-2 is in the cell serviced by base station 103-1, wireless switching center 120 returns the data message back to base station 103-1, which relays it to wireless terminal 101-2.
Because people can require emergency assistance (e.g., first aid, police, fire, etc.) when they travel, a wireless telecommunications system must be capable of processing an emergency call (e.g., a "911" call in the United States, a "999" call in Europe, etc.) that is initiated from a wireless terminal. In particular, a wireless telecommunications system must perform three main steps when processing an emergency call from a wireless terminal:
1. the system must ascertain the precise location of the wireless terminal (e.g., its latitude and longitude to within 100 feet); PA1 2. the system must route the emergency call to whichever emergency facility (e.g., police station, hospital, etc.) is closest to the location of the wireless terminal; and PA1 3. the system must provide the authorities at the emergency facility with the location of the wireless terminal. PA1 1) to follow a predetermined route, PA1 2) to remain in visual contact and play follow-the-leader, or PA1 3) to maintain an open telecommunications channel so that they can coordinate their movement on an ad hoc basis.
This last step is necessary because a person who places an emergency call from a wireless telephone might not know his or her precise location or might be injured or otherwise incapable of providing his or her location even if he or she knew it.
There are several techniques in the prior art for enabling a wireless telecommunications system to ascertain the precise location of a wireless terminal, and each requires the addition of specialized location equipment to the telecommunications system. Sometimes the location equipment is added to the base stations or other parts of the network, sometimes it is added to the wireless terminals, and sometimes it is added to both. In any case, the equipment for locating a wireless terminal is typically expensive.
Furthermore, although it is very important that a wireless telecommunications system be capable of processing an emergency call, the number of emergency calls actually placed is very small in comparison to the number of non-emergency calls. This has two significant implications. First, because the cost of the location equipment is expensive, the cost of the location equipment per emergency call is very high and, therefore, difficult to justify. Second, because emergency calls are infrequent, the location equipment is seldom used, which is an apparent waste of its unique capability.
Therefore, the need exists for a secondary use of the location equipment in a wireless telecommunications system, which secondary use does not interfere with its primary use in processing emergency calls and which helps to defray the cost of the location equipment.